1. Field of the Invention
The present invention relates to a position transducer for use as installed to the body of a machine tool, industrial equipment, precision measurement equipment or the like, each provided with a moving or movable part and a stationary part, which move linearly in relation to each other, to measure an amount of travel and moved position of the moving part relative to the stationary part.
2. Description of the Related Art
The position transducer is installed to a moving part and a stationary part of a machine tool, industrial equipment, precision measuring equipment or the like (will be referred to simply as xe2x80x9cmachine bodyxe2x80x9d hereunder), which move linearly in relation to each other, to measure an amount of travel and moved position of the moving part relative to the stationary part, for example.
The conventional position transducer includes a long graduated scale provided to detect a position during relative movement of the stationary and moving parts, and a sensor unit having disposed therein a detection head which reads a graduation provided on the scale. The scale is fixed to the stationary part while the sensor unit is fixed to the moving part. The sensor unit includes a slider disposed on the scale to read a displacement on the graduation by the detection head, and a carrier coupled to the slider to move along with the moving part of the machine body, the slider being coupled to the carrier by a coupling.
In the position transducer constructed as in the above, as the carrier is moved along with a linear movement of the moving part of the machine body, the slider is towed or pushed by the coupling connecting the slider and carrier, by the coupler, to the carrier and travels on and longitudinally of the scale. The position transducer reads, by the detection head disposed on the slider, a displacement of the slider on the graduation on the scale to measure an amount of travel and moved position of the moving part relative to the stationary part.
In the conventional position transducer, as the moving part of the machine body, installed as in the above, moves linearly, the carrier moves following up a meandering or swelling movement of the moving part of the machine body. Also, in the position transducer, the slider is disposed in contact with the scale and moves following up a surface condition of the scale, more specifically, a meandering or swelling of the scale surface. That is, in the position transducer, a relatively moved position and relative moving distance are measured by the slider and carrier destined for different movements, respectively, and so the positions of the slider and carrier are possibly deviated from each other in the course of a measurement. In the position transducer of this type, the slider and carrier are coupled to each other by applying a force to the slider or carrier independently of a contact point at the coupling connecting the slider and carrier. So, the above positional deviation cause a dynamic vector in a direction other than the moving direction of the slider, in other words, in a direction in which the linear movement of the slider is inhibited, which results in a measurement error due to a return error or degradation of repeatability precision. Generally, when the position transducer is constructed to attain a high performance and high resolution among others, a contact friction due to the dynamic vector developed at the coupling connecting the slider and carrier will inhibit the slider from moving linearly, which will influence the repeatability precision and return error.
To solve the above problem, it has been proposed to provide at the coupling connecting the slider and carrier a universal mechanism whose position is variable to accommodate an out-tolerance installation and out-tolerance part dimensions.
As one of such solutions to the above problem, a position transducer is known from the disclosure in the Japanese Published Unexamined Application No. 4-231810, in which a scanning unit which scans a scale plate is mounted with a coupling fixed in a measuring direction on a mount base of a machine tool, the coupling consisting of a first plate-shaped coupling piece fixed to the scanning unit and a second pin-shaped coupling piece fixed to a carriage and having a spheric end face, and a sliding layer is provided between the coupling pieces. In this position transducer thus constructed, an unmeasurable friction is generated between the coupling pieces to prevent degradation of measuring precision even when a relative movement takes place.
As another of such solutions, a position transducer is known from the disclosure in the Japanese Published Unexamined Application No. 5-87552, in which an intermediate coupling member is slidably coupled to a slider-side coupling member and the intermediate coupling and a carrier-side coupling member are put in line contact with each other in a concavo-convex relation to correct a detecting precision error.
In the position transducer disclosed in the Japanese Published Unexamined Application No. 5-87552, the intermediate coupling member and carrier-side coupling member are put in line contact with each other in the concavo-convex relation to form a universal coupling mechanism. In this universal coupling mechanism, the slider-side coupling member and intermediate coupling member are moved with a friction between them while being in surface contact because of the straightness of the machine vibration and travel and that of the scale. Namely, when the friction between the intermediate coupling member and carrier-side coupling member exceeds a one assured by the concavo-convex relation, the coupling members slide on each other with a change in angular relation between them, but since the frictional force changes if there is a difference in surface roughness between the coupling members, there occurs a difference in frictional force between the coupling members and so no stable coupling performance can be assured by such coupling members.
As mentioned above, in the position transducer disclosed in the Japanese Published Unexamined Application No. 4-231810, when the coupling consisting of the first plate-shaped coupling piece fixed to the scanning unit and the second pin-shaped coupling piece fixed to the carriage and having a spheric end face, has been used for a long time, a vibration-caused concavity will be formed in the first coupling piece to which the second coupling piece abuts and will possibly impair the stable coupling performance which would otherwise be assured.
Further, in a position transducer including a base unit by which a graduated scale is fixed to a base body, and a slider unit provided on the base unit slidably relative to the base unit and having a sensor which detects the graduation recorded on the scale, if not be perpendicular to a moving direction, a slider and carriage will be deviated from each other for the amplitude of a deflection caused by a machine motion and a swelling surface of the scale and the deflection will appear as a cosine error.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a position transducer including a long scale having a position detecting graduation provided thereon along the length thereof, a carriage moving along the scale and having a detection head which reads the graduation to provide a position detection signal, and a slider coupled to the carriage and moving the carriage along the length of the scale, in which when the slider and carriage are moved relative to each other, measurement error is reduced by eliminating Abbe error caused by a positional change taking place between coupling pieces.
The above object can be attained by providing a position transducer including a long scale having a position detecting graduation provided thereon along the length thereof, a carriage moving along the scale and having a detection head which reads the graduation to provide a position detection signal, and a slider coupled to the carriage and moving the carriage along the length of the scale; the carriage and slider are coupled to each other by an elastic member having an elasticity in a direction the carriage and slider are attracted towards each other, with the carriage and slider being butted to each other with a ball placed at one end of the carriage in the moving direction and between them.
With the above position transducer according to the present invention, with the carriage moving along the scale and having the detection head which reads the graduation to provide a position detection signal and the slider which moves the carriage along the length of the scale, being butted to each other with a rolling bearing placed at one end of the carriage in the moving direction and between them, the carriage and slider is coupled to each other by the elastic member having the elasticity in the direction in which the carriage and slider are attracted towards each other, whereby when the slider and carriage move relative to each other, it is possible to eliminate Abbe error due to a positional change occurring between the coupling pieces and thus assure a high accuracy measurement with a reduced measurement error.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.